FDISK(8) FreeBSD System Manager's Manual FDISK(8)
NAME
fdisk - PC slice table maintenance utility
SYNOPSIS
fdisk [-BIaipqstu] [-b bootcode] [-1234] [disk]
fdisk -f configfile [-itv] [disk]
PROLOGUE
In order for the BIOS to boot the kernel, certain conventions must be
adhered to. Sector 0 of the disk must contain boot code, a slice table,
and a magic number. BIOS slices can be used to break the disk up into
several pieces. The BIOS brings in sector 0 and verifies the magic
number. The sector 0 boot code then searches the slice table to
determine which slice is marked "active". This boot code then brings in
the bootstrap from the active slice and, if marked bootable, runs it.
Under DOS, you can have one or more slices with one active. The DOS
fdisk utility can be used to divide space on the disk into slices and set
one active.
DESCRIPTION
This command is obsolete. Users are advised to use gpart(8) instead.
The FreeBSD utility, fdisk, serves a similar purpose to the DOS utility.
The first form is used to display slice information or to interactively
edit the slice table. The second is used to write a slice table using a
configfile, and is designed to be used by other scripts/programs.
Options are:
-a Change the active slice only. Ignored if -f is given.
-b bootcode
Get the boot code from the file bootcode. Default is /boot/mbr.
-B Reinitialize the boot code contained in sector 0 of the disk.
Ignored if -f is given.
-f configfile
Set slice values using the file configfile. The configfile only
modifies explicitly specified slices, unless -i is also given, in
which case all existing slices are deleted (marked as "unused")
before the configfile is read. The configfile can be `-', in
which case standard input is read. See CONFIGURATION FILE,
below, for file syntax.
WARNING: when -f is used, you are not asked if you really want to
write the slices table (as you are in the interactive mode). Use
with caution!
-i Initialize sector 0 of the disk. Existing slice entries will be
cleared (marked as unused) before editing. (Compare with -u.)
-I Initialize sector 0 slice table for one FreeBSD slice covering
the entire disk.
-p Print a slice table in fdisk configuration file format and exit;
see CONFIGURATION FILE, below.
-q Be quiet. Benign warnings (such as "GEOM not found") are
suppressed.
-s Print summary information and exit.
-t Test mode; do not write slice values. Generally used with the -f
option to see what would be written to the slice table. Implies
-v.
-u Update (edit) the disk's sector 0 slice table. Ignored if -f is
given.
-v Be verbose. When -f is used, fdisk prints out the slice table
that is written to the disk.
-1234 Operate on a single slice table entry only. Ignored if -f is
given.
The final disk name can be provided as a "bare" disk name only, e.g. da0,
or as a full pathname. If omitted, fdisk tries to figure out the default
disk device name from the mounted root device.
When called with no arguments, it prints the sector 0 slice table. An
example follows:
******* Working on device /dev/ada0 *******
parameters extracted from in-core disklabel are:
cylinders=769 heads=15 sectors/track=33 (495 blks/cyl)
parameters to be used for BIOS calculations are:
cylinders=769 heads=15 sectors/track=33 (495 blks/cyl)
Warning: BIOS sector numbering starts with sector 1
Information from DOS bootblock is:
The data for partition 1 is:
sysid 165,(FreeBSD/NetBSD/386BSD)
start 495, size 380160 (185 Meg), flag 0
beg: cyl 1/ sector 1/ head 0;
end: cyl 768/ sector 33/ head 14
The data for partition 2 is:
sysid 164,(unknown)
start 378180, size 2475 (1 Meg), flag 0
beg: cyl 764/ sector 1/ head 0;
end: cyl 768/ sector 33/ head 14
The data for partition 3 is:
<UNUSED>
The data for partition 4 is:
sysid 99,(ISC UNIX, other System V/386, GNU HURD or Mach)
start 380656, size 224234 (109 Meg), flag 80
beg: cyl 769/ sector 2/ head 0;
end: cyl 197/ sector 33/ head 14
The disk is divided into three slices that happen to fill the disk. The
second slice overlaps the end of the first. (Used for debugging
purposes.)
sysid is used to label the slice. FreeBSD
reserves the magic number 165 decimal
(A5 in hex).
start and size fields provide the start address and
size of a slice in sectors.
flag 80 specifies that this is the active slice.
cyl, sector and head fields are used to specify the beginning
and end addresses of the slice.
Note: these numbers are calculated using BIOS's understanding of the disk
geometry and saved in the bootblock.
The -i and -u flags are used to indicate that the slice data is to be
updated. Unless the -f option is also given, fdisk will enter a
conversational mode. In this mode, no changes will be written to disk
unless you explicitly tell fdisk to.
The fdisk utility will display each slice and ask whether you want to
edit it. If you say yes, fdisk will step through each field, show you
the old value, and ask you for a new one. When you are done with the
slice, fdisk will display it and ask you whether it is correct. It will
then proceed to the next entry.
Getting the cyl, sector, and head fields correct is tricky, so by
default, they will be calculated for you; you can specify them if you
choose to though.
After all the slices are processed, you are given the option to change
the "active" slice. Finally, when all the new data for sector 0 has been
accumulated, you are asked to confirm whether you really want to rewrite
it.
The difference between the -u and -i flags is that the -u flag edits
(updates) the existing slice parameters while the -i flag is used to
"initialize" them (old values will be ignored); if you edit the first
slice, -i will also set it up to use the whole disk for FreeBSD and make
it active.
NOTES
The automatic calculation of starting cylinder etc. uses a set of figures
that represent what the BIOS thinks the geometry of the drive is. These
figures are taken from the in-core disklabel by default, but fdisk
initially gives you an opportunity to change them. This allows you to
create a bootblock that can work with drives that use geometry
translation under the BIOS.
If you hand craft your disk layout, please make sure that the FreeBSD
slice starts on a cylinder boundary.
Editing an existing slice will most likely result in the loss of all data
in that slice.
You should run fdisk interactively once or twice to see how it works.
This is completely safe as long as you answer the last question in the
negative. There are subtleties that fdisk detects that are not fully
explained in this manual page.
CONFIGURATION FILE
When the -f option is given, a disk's slice table can be written using
values from a configfile. The syntax of this file is very simple; each
line is either a comment or a specification, as follows:
# comment ...
Lines beginning with a # are comments and are ignored.
g spec1 spec2 spec3
Set the BIOS geometry used in slice calculations. There must be
three values specified, with a letter preceding each number:
cnum Set the number of cylinders to num.
hnum Set the number of heads to num.
snum Set the number of sectors/track to num.
These specs can occur in any order, as the leading letter
determines which value is which; however, all three must be
specified.
This line must occur before any lines that specify slice
information.
It is an error if the following is not true:
1 <= number of cylinders
1 <= number of heads <= 256
1 <= number of sectors/track < 64
The number of cylinders should be less than or equal to 1024, but
this is not enforced, although a warning will be printed. Note
that bootable FreeBSD slices (the "/" file system) must lie
completely within the first 1024 cylinders; if this is not true,
booting may fail. Non-bootable slices do not have this
restriction.
Example (all of these are equivalent), for a disk with 1019
cylinders, 39 heads, and 63 sectors:
g c1019 h39 s63
g h39 c1019 s63
g s63 h39 c1019
p slice type start length
Set the slice given by slice (1-4) to type type, starting at
sector start for length sectors. If the start or length is
suffixed with a K, M or G, it is taken as a Kilobyte, Megabyte or
Gigabyte measurement respectively. If the start is given as "*"
it is set to the value of the previous partition end. If the
length is given as "*" the partition end is set to the end of the
disk.
Only those slices explicitly mentioned by these lines are
modified; any slice not referenced by a p line will not be
modified. However, if an invalid slice table is present, or the
-i option is specified, all existing slice entries will be
cleared (marked as unused), and these p lines will have to be
used to explicitly set slice information. If multiple slices
need to be set, multiple p lines must be specified; one for each
slice.
These slice lines must occur after any geometry specification
lines, if one is present.
The type is 165 for FreeBSD slices. Specifying a slice type of
zero is the same as clearing the slice and marking it as unused;
however, dummy values (such as "0") must still be specified for
start and length.
Note: the start offset will be rounded upwards to a head boundary
if necessary, and the end offset will be rounded downwards to a
cylinder boundary if necessary.
Example: to clear slice 4 and mark it as unused:
p 4 0 0 0
Example: to set slice 1 to a FreeBSD slice, starting at sector 1
for 2503871 sectors (note: these numbers will be rounded upwards
and downwards to correspond to head and cylinder boundaries):
p 1 165 1 2503871
Example: to set slices 1, 2 and 4 to FreeBSD slices, the first
being 2 Gigabytes, the second being 10 Gigabytes and the forth
being the remainder of the disk (again, numbers will be rounded
appropriately):
p 1 165 63 2G
p 2 165 * 10G
p 3 0 0 0
p 4 165 * *
a slice
Make slice the active slice. Can occur anywhere in the config
file, but only one must be present.
Example: to make slice 1 the active slice:
a 1
FILES
/boot/mbr The default boot code.
SEE ALSO
boot0cfg(8), bsdlabel(8), gpart(8), newfs(8)
HISTORY
A version of fdisk first appeared in the Mach Operating System. It was
subsequently ported to 386BSD.
AUTHORS
fdisk for Mach Operating System was written by Robert Baron
<rvb@cs.cmu.edu>. It was ported to 386BSD by Julian Elischer
<julian@tfs.com>.
BUGS
The default boot code will not necessarily handle all slice types
correctly, in particular those introduced since MS-DOS 6.x.
The entire utility should be made more user-friendly.
Most users new to FreeBSD do not understand the difference between
"slice" and "partition", causing difficulty to adjust.
You cannot use this command to completely dedicate a disk to FreeBSD.
The bsdlabel(8) command must be used for this.
FreeBSD 13.1-RELEASE-p6 October 5, 2016 FreeBSD 13.1-RELEASE-p6
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